FD cast iron boilers are intended for burning coal, wood, and wooden briquettes. Boilers are assembled from high quality cast iron sections and for convenience the boiler's door is enlarged for easy loading of large fuel pieces. Non-electric automatic draft controller and manually regulated secondary air combustion control, provide air for complete burning. FD cast iron boilers are known for long service life even in low temperature operation. The boilers can be used for open or closed circulation heating systems, with open or pressure expansion vessels.